Vacuum tubes

ABSTRACT

An improvement is made in a known vacuum tube arrangement in which information is stored as a modulated electron stream traveling to and fro through a drift region between reflecting meshes, information retrieval being accomplished by gating pulses applied to one mesh to allow electrons to pass through it. Analysis of the gating mechanism shows that satisfactory highspeed operation requires a short electron transit time through the decelerating field associated with the gating mesh. One practical means of achieving this involves the provision of a high potential electrode between the drift region and the gating mesh.

United States Patent Inventors James Dwyer McGee;

Robin Wynclit'fe Smith, London, England Appl. No. 702,951 Filed Feb. 5, 1968 Patented Jan. 12, 1971 Assignee National Research Development Corporation London, England a corporation of Great Britain Priority Feb. 10, 1967 Great Britain VACUUM TUBES 5 Claims, 3 Drawing Figs.

US. Cl 315/16, 315/10, 315/12 Int. Cl l-l0lj 29/46 Field of Search 3 15/14- INPUT ULSE G ENE RATOR PULSE GENERATOR 2 DETECTOR TlMlNG CIRCUIT WAVE FORM GENERATOR [56] References Cited UNITED STATES PATENTS 2,276,320 3/1942 Linder 3 I5/15X 2,509,065 5/1950 Kirkpatrick 315/16 3,652,515 9/1953 McGee 315/15X Primary ExaminerRodney D. Bennett, Jr. Assistant Examiner-Malcolm F. Hubler AttorneyCushman, Darby & Cushman ABSTRACT: An improvement is made in a known vacuum tube arrangement in which information is stored as a modulated electron stream traveling to and fro through a drift region between reflecting meshes, information retrieval being accomplished by gating pulses applied to one mesh to allow electrons to pass through it. Analysis of the gating mechanism shows that satisfactory high-speed operation requires a short electron transit time through the decelerating field associated with the gating mesh. One practical means of achieving this involves the provision of a high potential electrode between the drift region and the gating mesh.

VACUUM TUBES This invention relates to vacuum tube arrangements of the kind incorporating: a vacuum tube comprising means for generating an electron stream of variable intensity, an output device spaced from the generating means andresponsive to electrons from the stream, a tubular electrode disposed between the generating means and the output device so as to be coaxial with a straight electron transmission path extending between the generating means and the output device, a first apertured screen disposed across said path between the generating means and the tubular electrode, and a second apertured screen disposed across said path between the tubular electrode and the output device; means for providing a focusing magnetic field extending along the transmission path; means for maintaining the tubular electrode at a positive potential with respect to the cathode of the tube so as to provide in the transmission path an elongated drift region along which electrons may travel with substantially uniform velocity; and means for individually switching the two screens between potentials which are respectively negative and positive with respect to the cathodeso as to control the flow of electrons along the transmission path.

Arrangements of this kind are described, for example, in a paper by J. D. McGee, J. Beesley and A. D. Berg, published in the Journal of Scientific Instruments, Volume 43, pages 153- 159 (Mar. 1966), and may be used for various purposes involving the storage of information. Thus, in operation of such an arrangement the electron stream is modulated in intensity in accordance with information which is to be stored, and the modulated stream is admitted into the space between the screens by maintaining the first screen at a positive potential. The electron stream may then be trapped in the space between the screens by maintaining both at negative potentials, the electron stream making repeated transits of the drift region in opposite directions'and being reflected on each approach to one or other of the screens by virtue of the decelerating field extending from the relevant screen in the direction of the drift region. When it is desired to retrieve an item of information from the tube, an appropriately timed gating pulse of short duration may be applied to the second screen so as to drive it positive, a corresponding part of the electron stream thereby being caused to emerge from the space between the screens and travel on to the output device.

Detailed analysis of the operation of such an arrangement has now shown that when a gating pulse is applied the sampling time (that is the period corresponding to that part of the original electron stream which is caused to pass through the second screen by the application of the gating pulse) is generally longer than the duration of the gating pulse. Moreover a dispersion effect is encountered due to variation of the length of the sampling time for different values of the velocity of entry of electrons into the decelerating field in front of the second screen, which will of course occur in practice because of the spread of emission energies of the electrons at the cathode. in order to make high speed operation feasible it is essential that this dispersion effect should be reduced as much as possible, and the present invention is concerned with the provision of means by which this may be achieved.

Broadly speaking the invention is based on the realization that the dispersion effect referred to can be reduced if the arrangement is made such as to obtain a relatively high value for the ratio of the magnitude of the decelerating field in front of the second screen to the distance over which this field extends; this corresponds to a relatively short electron transit time through the deceleration region.

The invention will be further explained with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of one vacuum tube arrangement of the kind specified, used as a framing camera;

PEG. 2 is an explanatory diagram; and

FIG. 3 is a diagrammatic view of part of the electrode structure of a vacuum tube for use in an arrangement of the kind specified, modified in accordance with the invention.

The arrangement shown in FIG. 1 incorporates a vacuum tube '1 having a tubular glassenvelope 2 which is closed at its ends by glass plates 3 and 4 respectively carrying a photocathode 5 and a phosphor screen 6; in operation the screen 6 is maintained at a high positive potential with respect to the cathode 5. The tube l is disposed coaxially' within a solenoid 7 which provides a strong axial'rnagnetic field such that electrons travelling along the length of the tube 1 will be constrained to follow helical paths of such small diameter that they may be regarded virtually as moving in straight lines parallel to the axis of the tube 1. The flow of electrons from the cathode 5 to the screen 6 is controllable by means of two apertured screens a and 9, which may suitably be in the form of wire meshes, the screen 8 being disposed adjacent the cathode 5 and the screen 9 being disposed at an intermediate position along the length of the envelope 2. In operation, the screen 8 is biassed slightly positive with respect to the cathode 5 but may be driven to a negative potential by pulses from a pulse generator 10, while the screen 9 is biassed slightly negative with respect to the cathode 5 but may be driven to a positive potential by pulses from a pulse generator 11, the generators 10 and 11 being controlled by a timing circuit 12 as explained further below.

Between the screens 8 and 9 is disposed a tubular electrode 13; constituted by a conductive coating on the wall of the envelope 2, which in operation is maintained at a positive potential (suitably about volts) with respect to the cathode 5 to provide an elongated drift region for electrons. Between the screen 8 and the electrode 13 is disposed a series of annular electrodes 14 which in operation are maintained by means of a potentiometer 15 at graded potentials such as to provide a substantially uniform electrostatic field between the screen 8 and the nearer end of the electrode 13.

Between the screen 9 and the screen 6 are disposed in succession an electrostatic deflection system 16 in the form of a pair of parallel plates, and an accelerating system comprising a series of annular electrodes 17 which in operation are maintained at suitably graded potentials by means of a potentiometer 18. A deflection waveform, of either ramp" or staircase" form, is arranged to be applied to the deflection system 16 from a deflection wave form generator 19 which is also controlled by the timing circuit 12.

Light from a high-speed event which is to be recorded is arranged to be imaged on to the photocathode 5 by means of a suitable optical system (not shown), the resultant electron stream emitted by the cathode 5 flowing through the screen 8 by virtue of the initial positive bias on the screen 8. Occurrence of the event is detected by a detector 20 having a rapid response, the output from which is arranged to initiate opera tion of the timing circuit 12. The operation of the latter is such that after a short delay a relatively long negative going pulse is applied to the screen 8 from the generator 10, so that the electron stream is'effectively trapped in the space between the screens 8 and 9, making repeated transits of this space in opposite directions. The timing circuit 12 also actuates the generators 11 and 19, the former being arranged to apply to the screen 9 a series of short positive going gating pulses so timed that a different sample from the electron stream will be allowed to pass through the screen 9 each time the stream approaches the screen 9. Each such sample will subsequently be accelerated to the phosphor screen 6 to produce an output image, the successive images (representing successive phases of the event to be recorded) appearing at different positions on the screen 6 by virtue of the deflection waveform applied to the deflection system 16 from the generator 19. The images appearing on the screen 6 are arranged to be photographed by means of a conventional camera (not shown). By using an arrangement as shown in FIG. 1, it is possible to accomplish high-speed photography with framing rates of up to 10 frames per second, with the camera synchronized to the event to be recorded.

As indicated in the general discussion above, the operation of the arrangement is vitally dependent on the manner in which sampling is effected by the gating pulses applied to the screen 9, and this point will now be considered in detail with reference to FIG. 2, which is an idealized diagram showing graphs of the variations of electrostatic potential and electron kinetic energy in the deceleration region between the electrode 13 and the screen 9, it being assumed for simplicity that there is a uniform electric field within this region. In the diagram, distance is measured from the end of the electrode 13 nearer the screen 9, the latter being disposed at a distance d; zero potential is taken to be that of the cathode 5 and the electrode 13 is assumed to be maintained throughout at a potential V The scales of electrostatic potential and electron kinetic energy are such that one volt on the former corresponds to one electron volt on the latter. The line a indicates the decelerating field which exists when the screen 9 is biassed negative by an amount V while the line b indicates the decelerating field which exists while a gating pulse is applied to the screen 9 so as to drive it positive by an amount V it being assumed that the fields are perfectly linear; in the analysis which follows it is assumed that all potentials change instantaneously, that is the gating pulse is assumed to be perfectly rectangular and any effects due to retarded potentials are neglected.

In general, electrons will enter the decelerating field from the drift region with kinetic energies slightly in excess of eV (where e is the electronic charge), due to their emission energies; we denote the voltage corresponding to the excess energy as V (which will typically have a value in the region ofzero to one volt). As they traverse the decelerating field towards the screen 9, the electrons will lose kinetic energy linearly with increasing distance at a rate dependent on the instantaneous value of the decelerating field; an electron will be reflected back into the drift region if its kinetic energy is reduced to zero before it reaches the screen 9 but will pass through the latter if it reaches it with finite kinetic energy, it being assumed that an accelerating field exists beyond the screen 9.

Consider first electrons which enter the decelerating field shortly before the application of a gating pulse. Where the conditions are such that the electron transit time through the deceleration region is relatively long, the variation of kinetic energy with distance as the electrons travel towards the screen 9 may be represented for three cases of electrons entering the decelerating field at successively later instants by paths such as ABCD, AEFG and in the diagram, the lines AHEB, CD, F0 and KL being parallel to the line a and the lines BC, EF and HK being parallel to the line b. It will be appreciated that the lines BC, EF and HK indicate the loss of kinetic energy for the relevant electrons during the gating pulse and that they increase in length in the order stated because of the different values of kinetic energy for the relevant electrons at the beginning of the gating pulse. The path ABCD corresponds to a case in which the electron enters the decelerating field sufficiently early to be reflected while the path AHKL corresponds to a case in which the electron enters the decelerating field sufficiently late to pass through the screen 9. The path AEFG corresponds to the limiting case of the first electron to enter the decelerating field which will pass through the screen. Where the electron transit time through the deceleration region is sufficiently short, however, an alternative possibility arises, namely that the limiting case is represented by the path AXG, where the line AX is parallel to the line a and the line X0 is parallel to the line B; in this case, the line XG will in general correspond to only part of the duration of the gating pulse. For either possibility we denote by the time which elapses from the instant at which the electron in the limiting case enters the decelerating field until the beginning of the gating pulse.

Consider now electrons which enter the decelerating field during the application of a gating pulse. The variation of kinetic energy with distance as the electrons travel towards the screen 9 may be represented for three cases of electrons entering the decelerating field at successively later instants by paths such as AMN, APG and ARS in the diagram, the lines MN, PG and RS being parallel to the line a and the line ARPM being parallel to the line B. The path AMN corresponds to a case in which the electron enters the decelerating field sufficiently early to pass through the screen 9, while the path ARS corresponds to a case in which the electron enters the decelerating field sufficiently late to be reflected. The path APG corresponds to the limiting case of the last electron to enter the decelerating field which will pass through the screen 9; we denote by t the time which elapses from the instant at which this electron enters the decelerating field until the end of the gating pulse.

The total sampling time T is therefore given by the equation:

where t is the length of the gating pulse.

Assuming that (as will normally be the case in practice) V V,, and V are all small compared with V it can be shown, by substituting for t, and t in equation (1), that m being the electronic mass; equations (2a) and (2b) are respectively applicable when T is greater and less than I/ It should be noted that T is approximately the electron transit time through the deceleration region which would be obtained if the screen 9 were maintained at zero potential. Consideration of equations (2a) and (212) indicates that as V increases from zero, T will increase monotonically, giving rise to the dispersion in sampling time. Clearly, the dispersion effect may be reduced, for given values oft, V and V by making the quantity VU/a' as large as possible, and therefore the quantity T as small as possible. With the arrangement described above it is not normally practicable to utilize very high values for V,,, the potential of the electrode 13, since this would unduly decrease the transit time of electrons in the drift region for a given length of this region. It will, therefore, normally be preferable to utilize a small value of d in order to bring about a reduction in the dispersion of sampling time. The improvement obtainable in this way may be illustrated by the following example, in which V V and V respectively have values of 100, 2 and 8 volts, and t has a value of 5 I nanoseconds. in this case, where d has a value of 5.5 centimeters, the value ofT when V is zero is 12.3 nanoseconds, while its value when V has a value of one volt is 16.9 nanoseconds, thus giving a spread in the sampling time of 4.6 nanoseconds; when a has a value of 0.55 centimeters, the value of T when V is zero is 5.9 nanoseconds, while its value when the value of V is 1 volt is 6.1 nanoseconds, thus giving a spread in the sampling time of 0.2 nanoseconds.

An approximation to the conditions discussed theoretically above may be achieved in practice by modifying the arrangement shown in FIG. 1 to incorporate a third apertured screen disposed across the end of the electrode 13 nearer to the screen 9, the third screen being maintained at the same potential as the electrode 13 and the spacing between the screen 9 and the third screen being made relatively small; from the point of view of obtaining a satisfactorily small dispersion of the sampling time, this spacing should not appreciably exceed one centimeter when the potential of the electrode 13 is volts which would correspond to a value of T of about 3.4 nanoseconds. The provision of the third screen however, has the disadvantage that the electrons must pass through it on each transit of the drift region, so that unless it has a very small shadow ratio the absorption of the electrons by the third screen severely restricts the number of transits which may be achieved in practice.

It is therefore desirable to consider alternative arrangements which do not involve the use of a screen extending across the drift region, and since in general these will not involve a linear decelerating field, it is convenient to define a figure of merit (comparable to the parameter T which is applicable generally to any given electrode system and set of operating conditions in a vacuum tube arrangement of the kind to which the invention relates. Thus denoting by V the potential (relative to cathode potential) at any point in the decelerating field when the second screen is biassed negatively, we define a point Z as that point on the axis of the electrode system at which the potential is increased to V 0.1 V,. when a gating pulse of magnitude V ,1 is applied to the second screen; the figure of merit T is then defined as equal to where V; is the value of V at the point Z and d is the distance from the point 2 to the second screen. It will be noted that for the linear case discussed above the value of T is approximately equal to 0.95T In general it would appear that in order to achieve a satisfactorily small dispersion of the sampling time the value of T; should not exceed 3.5 nanoseconds.

One arrangement which has proved satisfactory in practice for obtaining a low value of Tgil'lVOlVfiS the provision between the second apertured screen and the tubular electrode which defines the normal drift region of a second, relatively short, tubular electrode disposed coaxial with the first and maintained am much higher potential in order to provide a high value for the decelerating field in front of the second apertured screen. With this arrangement, the use of a plain tube as the high potential electrode will normally be satisfactory only if the electron flow is confined to a relatively small part of the cross section of the vacuum tube adjacent the axis, since although a relatively low value of T can be obtained it will be accompanied by a spread in the sampling time over the cross section of the tube due to the bulging of equipotentials into the interior of the second tubular electrode. This limitation can however readily be overcome by disposing across that end of the second tubular electrode nearer to the second apertured screen a metal plate maintained at the same potential as the second tubular electrode and having formed in it a long narrow slot which will restrict the bulging of the equipotentials.

One such arrangement isillustrated in FIG. 3, in which for convenience only that part of the'electrode structure is shown which is modified ascompared with the arrangement shown in FIG. 1. The apertured screen 9' and the tubular electrode 13' correspond respectively to the screen 9 and electrode 13 shown in FIG. 1, but between and spaced from them is disposed a further tubular electrode 21 having a diameter equal to that of the electrode l3'and having a length approximately equalto its diameter; the electrode 21 is disposed coaxial with the electrode 13, and the electrodes 13' and 21 may conveniently be constituted, as is the electrode 13 shown in F IG. 1, by conductive coatings formed on the wall of the envelope (not shown in FIG. 3). Across the end of the electrode 21 nearer the screen 9" is disposed a circular metal plate 22 connected to the electrode 21 and having formed in it a narrow slot 23 whose length extends along the major part of one diameter of the plate 22. In one particular example, where the diameter of the electrodes 13 and 21 is six centimeters, the spacing between the plate 22 and the screen 9' is two centimeters and the dimensions of the slot 23 are four centimeters by one centimeter, the electrode 13 being maintained at a positive potential of 100 volts and the electrode 21 and plate 22 being maintained at a positive potential of 2,000 volts. With this arrangement the figure of merit T has a value of about 1.7 nanoseconds, the spread in the sampling time being less than 0.2 nanoseconds and exhibiting virtually no variation over the area of the slot.

As an alternative to the arrangement just described the arrangement shown in FIG. 1 could be modified by incorporating a further apertured screen disposed immediately in front of the screen 9, the spacing between these two screens being very small (say a fraction of a millimeter) and the further screen being maintained throughoutat a small negative potential. In this case, when the screen 9 is biassed negative the plied to the screen 9, the positive equipotentials will penetrate through the further screen so as to allow electrons to pass through the two screens. With such an arrangement it would appear possible to achieve very small numerical values of the figure of merit T with a corresponding spread in the sampling time of the order of 0.1 nanoseconds; the arrangement would however have the disadvantage of involving a relatively high capacitance between the screen 9 and the further screen, which would restrict the use of high-speed pulses.

We claim:

1. A vacuum tube arrangement incorporating:

A. a vacuum tube having;

a. means, comprising an electron-emissive cathode, for

generating an electron stream of variable intensity, b. an output device spaced from the generating means and responsive to electrons from the stream, and c. an intermediate electrode system disposed between the generating means and the output device and including, 1. a tubular electrode disposed so as to be coaxial with a straight electron transmission path extending between the generating means and the output device, 2. a first apertured screen disposed across said path between the generating means and the tubular electrode, and 3. a second apertured screen disposed across said path between the tubular electrode and the output device;

B. means for providing a focusing magnetic field extending along said path;

C. means for maintaining the tubular electrode at a positive potential with respect to the cathode to provide in said path an elongated drift region along which electrons may travel with substantially uniform velocity;

D. means for selectively maintaining the first screen at a positive or negative potential with respect to the cathode;

E. means for biassing the second screen negative with respect to the cathode potential to establish a decelerating field for electrons emerging from said drift region in a direction towards the second screen, the potential at any point in the decelerating field relative to the cathode potential being V; and

F. means for applying to the second screen positive going gating pulses of magnitude V greater than the magnitude of the negative bias on the second screen; the arrangement being characterized by that improvement consisting of so dimensioning said decelerating field that the value of the parameter V, V,,, V,, m and e being expressed in a self-consistent system of units.

2. A vacuum tube arrangement according to claim 1, in which said intermediate electrode system further includes a second, relatively short, tubular electrode disposed coaxial with the first-mentioned tubular electrode between the latter and the second screen, the arrangement including means for maintaining the second tubular electrode at a much higher positive potential than the first.

3. A vacuum tube arrangement according to claim 2, in which there is disposed across that end of the second tubular electrode nearer to the second screen a metal plate maintained at the same potential as the second tubular electrode and having formed in it a long narrow slot.

which the spacing between th e second apertured screen and the adjacent end of the tubular electrode does not exceed one centimeter, there being disposed across this end of the tubular electrode a further apertured screen maintained at the same potential as the tubular electrode. a I 

1. A vacuum tube arrangement incorporating: A. a vacuum tube having; a. means, comprising an electron-emissive cathode, for generating an electron stream of variable intensity, b. an output device spaced from the generating means and responsive to electrons from the stream, and c. an intermediate electrode system disposed betweEn the generating means and the output device and including,
 1. a tubular electrode disposed so as to be coaxial with a straight electron transmission path extending between the generating means and the output device,
 2. a first apertured screen disposed across said path between the generating means and the tubular electrode, and
 3. a second apertured screen disposed across said path between the tubular electrode and the output device; B. means for providing a focusing magnetic field extending along said path; C. means for maintaining the tubular electrode at a positive potential with respect to the cathode to provide in said path an elongated drift region along which electrons may travel with substantially uniform velocity; D. means for selectively maintaining the first screen at a positive or negative potential with respect to the cathode; E. means for biassing the second screen negative with respect to the cathode potential to establish a decelerating field for electrons emerging from said drift region in a direction towards the second screen, the potential at any point in the decelerating field relative to the cathode potential being V; and F. means for applying to the second screen positive going gating pulses of magnitude Vp greater than the magnitude of the negative bias on the second screen; the arrangement being characterized by that improvement consisting of so dimensioning said decelerating field that the value of the parameter
 2. a first apertured screen disposed across said path between the generating means and the tubular electrode, and
 2. A vacuum tube arrangement according to claim 1, in which said intermediate electrode system further includes a second, relatively short, tubular electrode disposed coaxial with the first-mentioned tubular electrode between the latter and the second screen, the arrangement including means for maintaining the second tubular electrode at a much higher positive potential than the first.
 3. a second apertured screen disposed across said path between the tubular electrode and the output device; B. means for providing a focusing magnetic field extending along said path; C. means for maintaining the tubular electrode at a positive potential with respect to the cathode to provide in said path an elongated drift region along which electrons may travel with substantially uniform velocity; D. means for selectively maintaining the first screen at a positive or negative potential with respect to the cathode; E. means for biassing the second screen negative with respect to the cathode potential to establish a decelerating field for electrons emerging from said drift region in a direction towards the second screen, the potential at any point in the decelerating field relative to the cathode potential being V; and F. means for applying to the second screen positive going gating pulses of magnitude Vp greater than the magnitude of the negative bias on the second screen; the arrangement being characterized by that improvement consisting of so dimensioning said decelerating field that the value of the parameter
 3. A vacuum tube arrangement according to claim 2, in which there is disposed across that end of the second tubular electrode nearer to the second screen a metal plate maintained at the same potential as the second tubular electrode and having formed in it a long narrow slot.
 4. A vacuum tube arrangement according to claim 1, in which there is disposed between the tubular electrode and the second apertured screen a further apertured screen spaced by a very small distance from the second apertured screen and maintained at a small negative potential with respect to the cathode.
 5. A vacuum tube arrangement according to claim 1, in which the spacing between the second apertured screen and the adjacent end of the tubular electrode does not exceed one centimeter, there being disposed across this end of the tubular electrode a further apertured screen maintained at the same potential as the tubular electrode. 